1. Field of the Invention
This invention relates generally to electric devices. In particular, this invention relates to electric devices using superconducting materials. More particularly, this invention relates to a structure for a modulation device which uses superconducting materials as the active material.
2. Discussion of the Background
Metallic superconductors have been known since 1911. A physical description of this phenomena evolved throughout this century, including the London equations, the concepts of superconducting paired electrons for electrons near the Fermi surface of a metal, and the subsequent Bardeen Cooper Schrieffer (BCS) theory. A discussion of these theories and characterizations can be found in "INTRODUCTION TO SUPERCONDUCTIVITY", Robert E. Krieger Publishing Company, Malabra, Fla., by Michael Tinkham (Card Catalog number QC612.S8T49).
A more applied description of superconductivity as it relates to electronics is present in the book "ENERGY APPLICATIONS OF HIGH TEMPERATURE SUPERCONDUCTORS", Vol. 2, edited by S. J. Dale, S. M. Wolf and T. R. Schneider and published by Research Reports Center, Palo Alto Calif., and in Chapter 4 thereof, by C. H. Nowlin, Oak Ridge National Laboratory, which describes applications of superconductivity to power electronics which also includes a discussion of electric field switching of a superconducting thin film.
Chapter 4, by Nowlin et. al., indicates that superconducting field effect switches may have a reduced power loss than currently available switches and that electric motors and generators using superconducting switches would be considerably more efficient. A capacitor structure is described therein consisting of a metal-insulator-superconductor layer. Voltage applied to the metal electrode induces an electric field between the metal and superconducting electrodes.
The electric field induces a surface charge in a thin surface region of the superconducting layer and affects the total conduction charge in the superconducting layer. Thickness of the induced charge layer in the superconductor is proportional to n.sup.-1/6 where n is the carrier charge density. The induced charge only penetrates a few angstroms into the superconducting material. For n=5.times.10.sup.21 /cm.sup.3, as in YBa.sub.2 Cu.sub.3 O.sub.7 the induced charge extends about 5 angstroms, while for Cu the induced charge only penetrates about an angstrom. YBa.sub.2 Cu.sub.3 O.sub.7 has a perovskite type structure of the form ABO.sub.3 where Y and Ba are on the A sites, Cu is on the B sites, Oxygen is on the O sites, and some of the O sites are unoccupied.
The article "EXPERIMENTAL CONSIDERATIONS IN THE QUEST FOR A THIN FILM SUPERCONDUCTING FIELD-EFFECT TRANSISTOR", A. F. Hebard et. al., IEEE Transactions on Magnetics MAG-23 1279-1282, 1987, discusses results of electric field switching of superconducting layer of indium and indium oxide, a low superconducting transition temperature (T.sub.c) material with a T.sub.c of about 1 degree Kelvin, on a 1 micron thick ceramic dielectric layer. Only a fivefold modulation of the resistance of the switched layer was obtained. The transition temperature of these superconductors is low, around 1.0 degree Kelvin.
The article "FIELD EFFECT ON SUPERCONDUCTING SURFACE LAYERS OF SRTIO.sub.3 ", Extended Abstracts, Superconducting Materials, Proceedings of Symposium S, 1986, Fall Meeting of the Materials Research Society, pp. 47-49, discusses work on Ta doped SrTiO.sub.3 and Ba.sub.x Sr.sub.1-x TiO.sub.3 in which complete modulation of carrier concentration of these thin films was achieved. The carrier concentrations of these films is as low as 1.0.times.10.sup.20 carriers/cm.sup.3. Unfortunately, the superconducting transition temperature of these materials is also only about 1 degree Kelvin.
Recently, superconductivity in YBa2Cu3O7 was discovered. The high superconducting transition temperature of this material spurred work on related materials, leading to discovery of several structural families of metal-oxide superconductors whose properties are now well known. Each structural family is characterized by a crystal structure and in each family variations in physical properties, including superconducting properties, due to variation in composition of sublattices for the metal ions are well known. For example, in the '123" structure for YBa.sub.2 Cu.sub.3 O.sub.7-x in which x can vary between 0 and 1, superconductivity occurs when x is less than 0.6. Also, Y can be replaced by most of the other rare earths, including Nd, Gd, Sm and La without affecting the superconducting transition temperature, which is around 90 degrees Kelvin. Composition variations for the superconducting compounds having the structures of the following compounds: Tl.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10, Tl.sub.2 BA.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8, Tl.sub.2 Ba.sub.2 Cu.sub.1 O.sub.6, (TBCCO or TlBaCaCuO compounds) and the related Bi-Sr-Ca-Cu-0 and Bi-Sr-Cu-0 compounds (also called BSCCO or BiSrCaCuO compounds), the La.sub.2 CuO.sub.4 structure compounds (also known as 214 compounds and LaCuO compounds), the Ba-K-Bi-O (BKBO or BaKBiO) and Ba-Pb-Bi-0 (BPBO or BaPbBiO) structure compounds, are well known. Ba.sub.x K.sub.1-x BiO.sub.3 which is superconducting and has the perovskite structure is well known.
These metal oxide superconductors are well suited in comparison with other superconductors such as Pb and Nb for use as active materials in field effect devices for several reasons. Pb and Nb have carrier concentrations of roughly 10.sup.23 /cm.sup.3 so their screening length for induced surface charge is about 1 angstrom. Application of an electric field to a surface of a metal-oxide superconductor only modulates the charge density in the superconductor within a couple screening lengths of the surface. Since the free carrier charge density is on the order of 10.sup.21 carriers/cm.sup.3, the screening length is very short, only a few angstroms. The metal-oxide superconductors react with atmosphere to form nonsuperconducting surface layers.
The coherence length of superconductors decreases with increasing superconducting transition temperature. For materials having high superconducting transition temperatures, such as many of the metal-oxide superconductors, the coherence length of superconducting pairs is only 5 to 10 angstroms. When these pairs encounter a disordered area which is as large as their coherence length they tend to scatter and become unpaired. Therefore crystal imperfections, such as grain boundaries, drastically reduce the critical current in high superconducting transition temperature metal-oxide materials.
Thin films of many metal-oxide superconductors have been grown. However, these films have not exhibited stable electrical properties. The electrical characteristics of these films deteriorate rapidly with time. Also, for an electrically continuous thin film, a smooth substrate is necessary.
Some thin epitaxial films of metal-oxide superconductors have been deposited on single crystal substrates such as SrTiO.sub.3 and related perovskite materials. For very thin films to be electrically continuous, these substrates must also be very smooth. Various metal-oxide superconductor thin films on various substrates are described in the article in IEEE Transactions in Magnetics, 27, 2 (1991). In particular, this article describes thin film properties of thin films of YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO), Bi-Sr-Ca-Cu-O (BSCCO), Tl-Ba-Ca-Cu-0 (TSCCO) compounds including Bi.sub.2 (SrCa).sub.3 Cu.sub.2 O.sub.x, and surfaces and interfaces for Nd-Ce-Cu-0. This article also discusses multilayers of YBCO/insulator, YBCO/PBCO (PBCO is PrBa.sub.2 Cu.sub.3 O.sub.7, YBCO/MgO/YBCO, YBCO/Y.sub.2 BaCuO.sub.5, superconductor/insulator/superconductor structures and LaAlO.sub.3 /YBCO multilayers. The article in the Journal of Materials Research, published by the Materials Research Society, Pittsburg Pa., contains information in many articles on the properties of high temperature superconductor, ferroelectric, piezoelectric and insulator thin films. The book "COPPER OXIDE SUPERCONDUCTORS", by C. P. Poole, T. Datta, H. A. Farach, published by Wiley (1988) discloses different structural types and chemically equivalent substitutions in these structures which do not affect superconduction for CuO.sub.x superconductors. Chapter 4 of this book discloses thin film properties and Chapter 6 discloses crystallographic structure types, including the La.sub.2-x M.sub.x CuO.sub.4-y "214" structure and the RBa.sub.2 Cu.sub.3 O.sub.7-z "123" structure.
FIG. 19 shows a unit cell of the undistorted (cubic) perovskite structure, ABO.sub.3. The body centered atom 100, corresponds to A in the chemical formula, the edge centered atoms 101 correspond to the B atom in the chemical formula and the corner elements 102 correspond to the O atoms in the chemical formula. In the metal-oxide compounds O is oxygen.
FIG. 20 shows the La.sub.2 CuO.sub.4 structure. Oxygen is represented by the large open circles 104, La by the circles containing crosses 103, and Cu by small darkened circles 105. The crystallographic directions are indicated by the a, b and c axes.
FIG. 21 shows the YBCO structure. The large open circles 106 represent the body centered positions, containing Y and Ba and are labelled as such. The small dark circles 107 represent Cu atoms and the small open circles 108 represent oxygen atoms. The a, b, and c crystallographic directions are labelled.
FIG. 22 shows the various related layered structures of the BSCCO and TBCCO compounds. The numerals below each structure refer to the chemical formula of the compounds and the number of layers associated with each metallic element in the chemical formula. For example the 1122 structure corresponds to BiSr Ca.sub.2 Cu.sub.2 oxide and to TlBaCa.sub.2 Cu.sub.2 oxide. In FIG. 22 the large open circles represent Tl or Ba, the small open circles represent Ca, The cross-hatched circles represent Ba or Sr, the large dark circles represent Cu, and the small dark circles represent oxygen. These structures are illustrated in Solid State Physics-Superconductivity, Quasicrystals, TwoDimensional Phsyics, Vol. 42, Academic Press Pub., edited by H. Ehrenreich and D. Turnbull, page 135 to 211, San Diego.
FIG. 23 shows a diagram of one example of the lattice matching in an epitaxial structure in which a Si (100) surface is epitaxially contacted to yittria stabilized zirconia (YSZ) and the upper surface of the YSZ is epitaxially connected to the a-b plane of YBCO.
Several recent U.S. patents relating to oxide-superconductors are of interest and are hereby incorporated by reference; U.S. Pat. Nos. 5,034,374, 5,041,188, 5,019,551, 5,047,390 and 5,049,543.
Up until the present time field modulation of electrical properties of metal-oxide superconductors has not been practical due to the very thin films required and the problems with fabrication and compatibility of the materials involved in such structures.